feat: implement core markdown rendering pipeline

Add minimal e2e pipeline to transform content → HTML:

- src/error.rs: Custom error types with thiserror
- src/content.rs: YAML frontmatter parsing via gray_matter
- src/render.rs: Markdown → HTML via pulldown-cmark
- src/templates.rs: Maud templates for base layout and posts
- src/main.rs: Pipeline orchestrator

All 15 blog posts successfully rendered to public/blog/*/index.html.
Added thiserror and walkdir dependencies.
Added public/ and DepMap fragment to .gitignore and AGENTS.md.

src/main.rs

@@ -1,3 +1,66 @@
+//! nrd.sh - Bespoke static site compiler.
+//!
+//! Transforms markdown content into a minimal static site.
+
+mod content;
+mod error;
+mod render;
+mod templates;
+
+use crate::content::Content;
+use crate::error::{Error, Result};
+use std::fs;
+use std::path::Path;
+
 fn main() {
-    println!("nrd.sh compiler v{}", env!("CARGO_PKG_VERSION"));
+    if let Err(e) = run() {
+        eprintln!("error: {e}");
+        std::process::exit(1);
+    }
+}
+
+fn run() -> Result<()> {
+    let content_dir = Path::new("content");
+    let output_dir = Path::new("public");
+
+    if !content_dir.exists() {
+        return Err(Error::ContentDirNotFound(content_dir.to_path_buf()));
+    }
+
+    // For MVP: process all markdown files in content/blog/
+    let blog_dir = content_dir.join("blog");
+
+    for entry in walkdir::WalkDir::new(&blog_dir)
+        .into_iter()
+        .filter_map(|e| e.ok())
+        .filter(|e| {
+            e.path().extension().map_or(false, |ext| ext == "md")
+                && e.path().file_name().map_or(false, |n| n != "_index.md")
+        })
+    {
+        let path = entry.path();
+        eprintln!("processing: {}", path.display());
+
+        let content = Content::from_path(path)?;
+        let html_body = render::markdown_to_html(&content.body);
+        let page = templates::render_post(&content.frontmatter, &html_body);
+
+        let out_path = output_dir.join(content.output_path(content_dir));
+        let out_dir = out_path.parent().unwrap();
+
+        fs::create_dir_all(out_dir).map_err(|e| Error::CreateDir {
+            path: out_dir.to_path_buf(),
+            source: e,
+        })?;
+
+        fs::write(&out_path, page.into_string()).map_err(|e| Error::WriteFile {
+            path: out_path.clone(),
+            source: e,
+        })?;
+
+        eprintln!("  → {}", out_path.display());
+    }
+
+    eprintln!("done!");
+    Ok(())
 }